1. Field of the Invention
It is common to measure Aitken nuclei (airborne particles in the approximate size range of 10.sup.-7 to 10.sup.-5 centimeters radius) by the condensation of water vapor on the particles in a cloud chamber to produce optically detectable droplets of several micrometers in diameter (1 to 10 .times. 10.sup.-4 cm) which are either photographed for counting or measured photometrically by measuring the intensity of scattered light or the attenuation of light passed in a fixed volume. A standard technique used in Aitken nuclei counters is to humidify the sample of gas to be tested to approximately the saturation point and subject the sample to an adiabatic expansion to lower the temperature to well below the dew point causing the excess water vapor of the super-saturated sample to condense upon any particles present. It has been established through experimentation that droplet growth cross-sectionally (radius squared) is essentially linear with time as long as excess water vapor from the supersaturated condition is available. Growth stops when the super-saturated condition ceases to exist. In photometric measurement by measurement of the scattering of light, it is common to use a dark field optical system, illuminating the cloud chamber with light and using a photodetector to view the light scattered by the droplet formed. The light source, the photodetector and baffles are placed in the chamber in such a way that the photodetector sees no light directly from the source and thereby measures only the intensity of scattered light.
2. Description of the Prior Art
The prior art is well represented by the device of Skala and Rich as described in U.S. Pat. No. 3,203,309, issued Aug. 31, 1965, which is made up of a cloud chamber adapted for the repetitious purging, charging with gas samples, adiabatic expansion and the measurement of light scattered by droplet growth about the Aitken nuclei and particle measurement means, as for example electronic circuitry, which converts the intensity of the scattered light to an electrical output signal at some predetermined stage of the particle growth for each gas sampling. Generally, the prior art read-out circuitry produces a reading on an instrument, such as a voltmeter, of a peak value obtained or of a reading at a particular time. They may also include means for comparison of readings with a reference standard and they may include other features such as means providing for step functions as disclosed in Skala and Rich. It appears that the art as represented by Skala and Rich and also by Skala, U.S. Pat. No. 3,010,308; VanLuik, U.S. Pat. No. 3,011,390, Rich, U.S. Pat. No. 2,956,435; and others displays a need for a more sophisticated measurement system which will provide for wide range linearity without sacrificing sensitivity.